reexec: add reexectest package

This package allows using the reexec functionality to execute child
processes as part of a test.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

reexec/reexectest/reexectest.go

@@ -0,0 +1,110 @@
+// Package reexectest provides small helpers for subprocess tests that re-exec
+// the current test binary. The child process is selected by setting argv0 to a
+// deterministic token derived from (t.Name(), name), while -test.run is used to
+// run only the current test/subtest.
+//
+// Typical usage:
+//
+//	func TestSomething(t *testing.T) {
+//		if reexectest.Run(t, "child", func(t *testing.T) {
+//			// child branch
+//		}) {
+//			return
+//		}
+//
+//		// parent branch
+//		cmd := reexectest.CommandContext(t, t.Context(), "child", "arg1")
+//		out, err := cmd.CombinedOutput()
+//		if err != nil {
+//			t.Fatalf("child failed: %v\n%s", err, out)
+//		}
+//	}
+package reexectest
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"os"
+	"os/exec"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+const argv0Prefix = "reexectest-"
+
+// argv0Token returns a short (16 hex chars) deterministic argv0 token
+// based on the test's name to prevent collisions.
+func argv0Token(t *testing.T, name string) string {
+	sum := sha256.Sum256([]byte(t.Name() + "\x00" + name))
+	return argv0Prefix + hex.EncodeToString(sum[:8])
+}
+
+// Run runs f in the current process iff it is the matching child process for
+// (t, name). It returns true if f ran (i.e., we are the child).
+//
+// When Run returns true, callers should return from the test to avoid running
+// the parent branch in the child process.
+func Run(t *testing.T, name string, f func(t *testing.T)) bool {
+	t.Helper()
+
+	if os.Args[0] != argv0Token(t, name) {
+		return false
+	}
+
+	// Scrub the "-test.run=<pattern>" that was injected by CommandContext
+	origArgs := os.Args
+	if len(os.Args) > 1 && strings.HasPrefix(os.Args[1], "-test.run=") {
+		os.Args = append(os.Args[:1], os.Args[2:]...)
+		defer func() { os.Args = origArgs }()
+	}
+
+	f(t)
+	return true
+}
+
+// Command returns an [*exec.Cmd] configured to re-exec the current test binary
+// as a subprocess for the given test and name.
+//
+// The child process is restricted to run only the current test or subtest
+// (via -test.run). Its argv[0] is set to a deterministic token derived from
+// (t.Name(), name), which is used by [Run] to select the child execution path.
+//
+// On Linux, the returned command sets [syscall.SysProcAttr.Pdeathsig] to
+// SIGTERM, so the child receives SIGTERM if the creating thread dies.
+// Callers may modify SysProcAttr before starting the command.
+//
+// It is analogous to [exec.Command], but targets the current test binary.
+func Command(t *testing.T, name string, args ...string) *exec.Cmd {
+	return commandContext(t, t.Context(), name, args...)
+}
+
+// CommandContext is like [Command] but includes a context. It uses
+// [exec.CommandContext] under the hood.
+//
+// The provided context controls cancellation of the subprocess in the same
+// way as [exec.CommandContext].
+//
+// On Linux, the returned command sets [syscall.SysProcAttr.Pdeathsig] to
+// SIGTERM. Callers may modify SysProcAttr before starting the command.
+func CommandContext(t *testing.T, ctx context.Context, name string, args ...string) *exec.Cmd {
+	return commandContext(t, ctx, name, args...)
+}
+
+func commandContext(t *testing.T, ctx context.Context, name string, args ...string) *exec.Cmd {
+	t.Helper()
+	exe, err := os.Executable()
+	if err != nil {
+		t.Fatalf("os.Executable(): %v", err)
+	}
+
+	argv0 := argv0Token(t, name)
+	pattern := "^" + regexp.QuoteMeta(t.Name()) + "$"
+
+	cmd := exec.CommandContext(ctx, exe)
+	cmd.Path = exe
+	cmd.Args = append([]string{argv0, "-test.run=" + pattern}, args...)
+	setPdeathsig(cmd)
+	return cmd
+}

reexec/reexectest/reexectest_linux.go

@@ -0,0 +1,14 @@
+//go:build linux
+
+package reexectest
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+func setPdeathsig(cmd *exec.Cmd) {
+	if cmd.SysProcAttr == nil {
+		cmd.SysProcAttr = &syscall.SysProcAttr{Pdeathsig: syscall.SIGTERM}
+	}
+}

reexec/reexectest/reexectest_other.go

@@ -0,0 +1,7 @@
+//go:build !linux
+
+package reexectest
+
+import "os/exec"
+
+func setPdeathsig(*exec.Cmd) {}

reexec/reexectest/reexectest_test.go

@@ -0,0 +1,101 @@
+package reexectest_test
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"reflect"
+	"strings"
+	"testing"
+
+	"github.com/moby/sys/reexec/reexectest"
+)
+
+func TestRun(t *testing.T) {
+	t.Run("env-and-output", func(t *testing.T) {
+		const expected = "child-env-and-output-ok"
+		if reexectest.Run(t, "env-and-output", func(t *testing.T) {
+			if got := os.Getenv("REEXEC_TEST_HELLO"); got != "world" {
+				t.Fatalf("env REEXEC_TEST_HELLO: got %q, want %q", got, "world")
+			}
+			fmt.Println(expected)
+		}) {
+			return
+		}
+
+		cmd := reexectest.CommandContext(t, t.Context(), "env-and-output")
+		cmd.Env = append(cmd.Environ(), "REEXEC_TEST_HELLO=world")
+
+		out, err := cmd.CombinedOutput()
+		if err != nil {
+			t.Errorf("env-and-output child failed: %v\n%s", err, out)
+		}
+		if got := strings.TrimSpace(strings.TrimSuffix(string(out), "PASS\n")); got != expected {
+			t.Errorf("env-and-output output: got %q, want %q", got, expected)
+		}
+	})
+
+	t.Run("exit-code", func(t *testing.T) {
+		if reexectest.Run(t, "exit-code", func(t *testing.T) {
+			os.Exit(23)
+		}) {
+			return
+		}
+
+		cmd := reexectest.CommandContext(t, t.Context(), "exit-code")
+		err := cmd.Run()
+		if err == nil {
+			t.Fatalf("expected non-nil error")
+		}
+
+		var ee *exec.ExitError
+		if !errors.As(err, &ee) {
+			t.Fatalf("got %T, want *exec.ExitError", err)
+		}
+		if code := ee.ProcessState.ExitCode(); code != 23 {
+			t.Fatalf("exit code: got %d, want %d", code, 23)
+		}
+	})
+
+	t.Run("args-passthrough", func(t *testing.T) {
+		const expected = "child-args-passthrough-ok"
+		if reexectest.Run(t, "args-passthrough", func(t *testing.T) {
+			want := []string{"hello", "world"}
+			got := os.Args[1:]
+			if !reflect.DeepEqual(got, want) {
+				t.Fatalf("args: got %q, want %q (full os.Args=%q)", got, want, os.Args)
+			}
+			fmt.Println(expected)
+		}) {
+			return
+		}
+
+		cmd := reexectest.CommandContext(t, t.Context(), "args-passthrough", "hello", "world")
+		out, err := cmd.CombinedOutput()
+		if err != nil {
+			t.Errorf("args-passthrough child failed: %v\n%s", err, out)
+		}
+		if got := strings.TrimSpace(strings.TrimSuffix(string(out), "PASS\n")); got != expected {
+			t.Errorf("args-passthrough output: got %q, want %q", got, expected)
+		}
+	})
+}
+
+func TestRunNonSubtest(t *testing.T) {
+	const expected = "child-non-sub-test-ok"
+	if reexectest.Run(t, "non-sub-test", func(t *testing.T) {
+		fmt.Println(expected)
+	}) {
+		return
+	}
+
+	cmd := reexectest.CommandContext(t, t.Context(), "non-sub-test")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Errorf("non-sub-test child failed: %v\n%s", err, out)
+	}
+	if got := strings.TrimSpace(strings.TrimSuffix(string(out), "PASS\n")); got != expected {
+		t.Errorf("non-sub-test output: got %q, want %q", got, expected)
+	}
+}